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Pulse plasma nitriding systems deliver uniform case-hardening and increased speed of processing when treating high wear parts.

Did you think purple was just a fashion statement? Explore the advantages, applications, and future of advanced pulse plasma nitriding in this Technical Tuesday. Heat Treat Today hopes you enjoy this original content piece by technical writer Jeff Elliott in cooperation with PVA TePla.

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When case-hardening the surface of steel, or steel alloy parts, commonly used in the transportation industry, such as gears, crank pins, dies, camshafts, the options have traditionally included one-of-three processes: carburizing, salt-bath nitrocarburizing and gas nitriding.  Each process has advantages and disadvantages, but those seeking more precise control of the diffusion layer formation, depth of case hardening, and preservation of component dimensions are increasingly turning to plasma nitriding.

Although pulse plasma has been utilized for decades, advanced pulse plasma nitriding offers absolute control of the DC (direct current) pulsing signal.  In addition, improved chamber design and construction allow for more precise temperature control and uniform distribution of the heat zone throughout the hot-wall chamber. The result is extremely consistent and uniform nitriding batch-to-batch and part-to-part, with less gas consumption.

“The benefits are more precise control of the diffusion layers, and its broader appeal to heat treat more diverse materials, beyond steel, that include titanium, stainless steel, and even aluminum,” says Thomas Palamides, senior product and sales manager at PVA TePla America.

In addition, commercial heat treat shops and high-volume part producers can now select from multiple system configurations that offer flexibility, efficiency, repeatability, and throughput optimization. As a result, global manufacturers in machined parts, tool design, die forming, die cutting, medical device manufacturing, additive manufacturing, electric vehicles, trains, electric generators, and land-based power systems are now leveraging these systems to run a cleaner, more efficient operation.

Pulse Plasma Nitriding Advantages

For steel and steel alloys, case-hardening can be achieved by carburizing, nitriding, cyaniding, or carbonitriding. Although carburizing is a traditional approach, the part has to be raised above the A3 temperature (727°C or 1341°F) on the Iron-Carbon diagram, usually in the temperature range of 900-930°C (1652-1706°F) or higher. Since the solubility of carbon is higher in the austenitic state, than the ferritic state, a fully austenitic state is required for carburizing.

Along with the high temperatures and time-at-temperature associated with carburizing, parts can be distorted. Depending on the part, and its geometric tolerances, limited machining may also be required.

An alternative to carburizing is nitriding, a lower-temperature, time-dependent, thermo-chemical process used to diffuse nitrogen into the surface of metal.

One method is salt bath nitriding. In this process, liquid immersion is required, and is typically conducted at 550 to 570°C (1022 to 1058°F). The source of nitrogen is a nitrogen-containing salt, such as sodium cyanide, often greater than 50% in concentration. However, with salt bath nitriding, post-bath cleaning is required to remove the residual cyanide-based treatment. In addition, there are disposal costs for salt and washing , also known as sodium hydroxide, environmental handling costs, as well as safety and operational liabilities.

Gas nitriding (500^oC or 932°F) and gas nitrocarburizing (540-580^oC or 1004-1076°F) are universally accepted procedures, and typically require a high concentration of ammonia (NH3), and a high amount of carrier gas flow (normal pressure process) compared with pulse plasma nitriding.  The elemental nitrogen gas constituent diffuses into iron and forms hard nitrides.  Because of the reduced temperature compared to carburizing, no quenching is necessary, and therefore the chance for distortion and cracking are lower.

Several disadvantages of gas nitriding are that it requires the use of flammable gases like ammonia, and high gas consumption compared to pulse plasma.  Gas nitriding is also not able to treat rust- and acid-resistant steels (i.e. stainless steels with greater than 12% Cr content) due to the impenetrable layer of the protective surface oxide layer.  Where as the energized pulsed plasma signal, during heat up, allows for the dissolution, or breakdown, of this thin protective layer, in effect cleaning the surface, allowing atomic nitrogen to penetrate.

With recent advancements in pulse plasma nitriding, however, a new level of precision and control is possible which results in uniform and consistent case hardening. Together with the advantages of using environmentally friendly gases only – in contrast to the use of ammonia in gas nitriding – plasma-based nitriding has become a focal point for additional innovations and a requirement for those that seek a more environmentally and safe solution.

In pulse plasma nitriding, parts are loaded into a heated vacuum chamber. After evacuating the chamber to a working pressure of 50 to 400 Pa, on a supporting fixture, to be covered by a bell chamber.  The chamber is evacuated to below 10 Pa (7.5 x 10^-2 Torr) prior to heating and a pulsating DC voltage of several hundred volts is applied between the charge (cathode) and the chamber wall (anode).  The process gas in the chamber is then ionized and becomes electrically conducting. For this type of process, nitrogen and hydrogen gas mixtures and gases with carbon additions, like methane are often utilized.

Depending on treatment time and temperature, nitrogen atoms diffuse into the outer zone of components and form a diffusion zone. This can be is atomic nitrogen, dissolved in the iron lattice, as well as in the form of included nitrate deposition.

Adding further precision, innovators in advanced pulse plasma have discovered methods to optimize the process through better control of the pulses. In the PulsPlasma® process developed by PVA TePla AG Industrial Vacuum Systems, for example, a precision regulated gas mixture of nitrogen, hydrogen, and carbon-based methane is used. A pulsating DC voltage signal of several hundred volts is delivered in less than 10 microseconds per pulse to ionize the gas. This serves to maximize the time between pulses for superior temperature control throughout the chamber.

“If you have a temperature variance of plus-minus 10 degrees within a batch, you will get completely different treatment results,” says Dietmar Voigtländer, sales manager at PlaTeG – Product Group with PVA Industry Vacuum Systems (IVS), Wettenberg, Germany, the manufacturer of PulsPlasma nitriding systems. “However, by controlling the pulse current by means of an exact pulse on and off time management, the overall temperature can be precisely managed with a uniform distribution, from top to bottom, throughout the hot wall chamber.”

A unique feature with this approach is that the system can be switched on to a stable glow discharge at room temperature. Most systems cannot do this because the generators do not supply stable plasma. To compensate, those systems must first be heated to 300-350°C (572-662°F) before plasma can be applied, adding time to the process.

[blocktext align="left"]“The benefits are more precise control of the diffusion layers, and its broader appeal to heat treat more diverse materials, beyond steel, that include titanium, stainless steel, and even aluminum." -Thomas Palamides, PVA TePla America.[/blocktext]Even the materials of construction used to manufacture the nitriding systems furnace itself have been optimized. In all systems, PlaTeG uses insulative materials developed in the aerospace industry to create a furnace wall as thin as 40 millimeters, compared to the industry standard of 150 millimeters. With less wall mass, the furnace requires less energy and time to heat, while still protecting workers that may accidentally touch the outside of the chamber.

With better overall control, advanced pulse nitriding furnaces offer multiple heating and cooling zones with each controlled by its own thermocouple. “This will create a very uniform temperature distribution within plus or minus 5 degrees Celsius (9 degrees Fahrenheit) from the bottom to the top of the furnace,” said Voigtländer.

Uniformity of temperature within a chamber pays a dividend beyond the consistency of nitriding results. With an even temperature throughout the chamber, the entire space is available for loading components which effectively increases the chamber’s capacity.

Stainless Steel – a Softer Steel

One of the key advantages of pulse plasma nitriding is that it is more suited to heat treating of high alloy materials such as stainless steel. When working with steels that have a higher chromium content, liquid nitriding can react with chromium and other elements, resulting in a loss of corrosion resistance.

Stainless steel has a natural passivation layer of chromium oxide, which inhibits corrosion. To bring nitrogen into the material, the chromium oxide layer must first be removed. With gas nitriding, removal of the passivation layer requires the application of a special gas chemistry, stainless steels can also be nitrided in salt baths, but only with a sacrifice of some level of corrosion resistance.

In the case of PulsPlasma nitriding, the treatment is applied directly through controlled ionic bombardment of the surface. By choosing a nitriding temperature below 450°C (or 842°F), and with exact control of the gas mixture, the material surface can be treated without reducing the corrosion resistance of the material.

Dies, Stamping & Injection Molds – a Harder Steel

Today, various molds and dies are used to shape everything from plastic bottles, to automobile quarter panels, to extruded wire, to metal injection molded (MIM) parts. Depending on the intricacy of the mold and die, it can cost a customer hundreds of thousands of dollars to fabricate. Customers require die longevity, while maintaining part tolerance, throughout the life of the die to ensure return-on-investment (ROI).

Despite being made of hardened steel, however, injecting melted resins at high temperatures and pressure into cavities over thousands of cycles begins to wear away and erode die edges, cavities, and moving components. Even the thermoplastic material can be abrasive, acting like sandpaper or leaving residue that wears down the surface. With the ever-increasing utilization of even more abrasive material, in the form of long glass and composite fibers, the amount of abrasion and friction within molds is increasing.

According to Voigtländer, pulse plasma nitriding is an ideal solution both to protect molds against damage and corrosion, but also because the diffusion of nitrogen increases the lubricity of the surface, facilitating quick removal of parts. “The diffusion of the nitrogen into the mold surface," he explains, "increases the fatigue strength of the material. In doing so, you can protect the surface against scratches… and increase the lifetime of the mold or die."

Sintered Parts

Pulse plasma nitriding also represents a strong option for sintered, or sinter hot isostatic pressing (HIP) components manufactured though additive manufacturing.

Salt baths have historically been used to nitride sintered components. However, the process of immersing components in molten salt makes it very difficult to remove the salt from open surface porosity. When gas nitriding is used, the nitriding atmosphere permeates all open pores resulting in the entire component, the surface, and the core, being completely nitrided.

“With pulse plasma nitriding, the atmosphere goes through the pores and only the surface area is affected, leaving the core or base material soft,” said PVA’s Voigtländer. “Most designers prefer having a wear-resistant surface with a soft, elastic core.”

Increased Production Throughput

Nitriding is a batch process. Innovation in furnace design, through an optimized mechanical operation, can increase efficiency and increase production capacity. While the actual time for nitriding does not change, efficient loading and unloading scenarios plays an important part. The PlaTeG plant design can use any one of a Mono, Shuttle or Tandem footprint, to manage throughput, resources, and operations costs.

As a batch process, nitriding typically requires waiting for the prior batch to be treated, cooled, and unloaded before a new batch can be started. Shuttle and tandem extensions are now available to streamline the batch process.

With a shuttle extension, an additional vacuum chamber bottom may be added to a furnace. During a running nitriding process, the unloading of an earlier batch and the loading/preparing of a subsequent batch on the second vacuum chamber is possible. The cycle time therefore for two consecutive batches is reduced because of the overlapping of the time for unloading/loading of a vacuum chamber with the treatment time of the running process.

With a tandem extension there are two complete vacuum chambers which are operated alternately by the vacuum pumps, the process gas supply, the plasma generator and the control unit of the system. In situations such as unmanned weekend operations, an automatic process can be started and controlled for both batches in succession. With this type of operational structure, “it is possible to increase overall nitriding capacity by 30-60% annually,” according to Voigtländer.

Because plasma nitriding uses environmentally friendly nitrogen and hydrogen, the furnaces can be collocated with the machining of components without requiring a separate room. Moreover, the pulse plasma nitriding systems produce no polluting gases. This makes nitriding more efficient as part of an overall manufacturing process as an operator can locate the furnaces beside their drilling machines.

Pulse plasma offers significantly more precision in nitriding through the control of the mixture of gases, the controllability of glow discharge intervals, the design of the Pulsed signal, and the use of a highly insulated hot wall nitride furnace. Together with innovations in the design of the furnaces to streamline batch management in nitriding operations, manufacturers who depend on nitriding components can benefit from greater uniformity of results, better-protected materials, and increased throughput.

 

About the Author: Jeff Elliott is a Torrance, California-based technical writer. He has researched and written about industrial technologies and issues for the past 20 years. He wrote this article in cooperation with PVA TePla.